SRA

The blacklegged tick, Ixodes scapularis, is a vector of multiple pathogens to humans, and domestic animals, including Borrelia burgdorferi sensu stricto, the causative agent of Lyme disease in the U.S. There are well-known geographic differences in Lyme disease incidence, tick questing behaviors, tick host use, and B. burgdorferi s.s. prevalence in these ticks. Despite the importance of I. scapularis and the need for detailed genetic information to inform control strategies, previous phylogeographic studies of these ticks have been restricted to relatively few populations or few genetic loci. Here we present the most comprehensive phylogeographic study of I. scapularis conducted to date by using double-digest RADseq and surveying 353 ticks collected from 33 counties within 22 states throughout the range of I. scapularis. We found limited genetic variation among populations from the Northeast and Upper Midwest, where Lyme disease is most common, and higher genetic variation among populations from the South. We identify four genetic clusters of I. scapularis that are consistent with four major geographic regions, plus a distinct Central Florida group. In regions where Lyme disease is currently increasing in frequency, such as Ohio, western North Carolina and eastern Tennessee, the I. scapularis populations genetically group with the northeastern ticks, where Lyme disease is highly endemic. Finally, we identify the top 10 variable DNA sites surveyed that contribute the most to population differentiation. These variable DNA sites cluster on one of the chromosome-scale scaffolds in the latest I. scapularis genome assembly and are all inside identified genes. Our findings illuminate additional research that is needed to identify loci causing variation in the vectorial capacity of I. scapularis and where additional tick sampling would be most valuable to further understand disease trends caused by pathogens transmitted by I. scapularis.